Existing hydroforming processes use high-pressure hydraulic fluid to compress a piece of working material against a stationary die or moving punch to form an article of a particular shape. It is known in the art to use a hydroforming process to shape complex structures from pieces of sheet metal or composite materials, for example continuous-fiber or woven-fiber composites. Due to the even application of hydrostatic pressure supplied by the hydroforming process, it is known to provide the advantage of reducing the presence of wrinkles and folds in the shaped working material.
Formation of a multi-layered continuous-fiber or woven-fiber composite requires a curing step wherein pressure and heat are applied to the layers of composite material in order to compress and fuse together the resin base of the layers of composite material, remove air voids from between the layers, and finally cure or set the layers of composite material together. However, typical fiber materials suitable for ballistic composite sheets, for example polyethylene, are known to degrade when subject to high heat for an extended period of time.
In addition, the known benefits of using a hydroforming process to shape composite structures allows the user to avoid placing relief cuts into the body of the flat layers of composite material prior to the forming process, which is necessary when using other known shaping methods in order to reduce wrinkling of the layers of composite material during the formation process. A drawback to placing relief cuts into the flat layers is that the strength of the layers of composite material, in particular the strength of the reinforcing fibers thereof, is reduced when the composite layers (i.e., the reinforcing fibers) are cut. Thus, it is desirable to avoid cutting into the body of the layers of composite material in order to maximize the strength of these layers. The use of a hydroforming process during the pre-forming stage makes avoidance of cutting into the body of the flat composite layers possible. The prior art also discloses mechanical clamping of the working material within the chamber of the hydroform press prior to the pressurization step in order to prevent wrinkling of the working material during the hydroforming process.
Therefore, it is desirable to have a process for forming or shaping articles which improves the formed shape of the article, while reducing the duration of exposure to heat experienced by the reinforcement fiber materials and avoiding the need to provide relief cuts into the body of the layers of the composite material, all while not significantly impacting the costs associated with production of the final article.
Relevant background references include U.S. Pat. Nos. 5,578,158, 6,631,630, and 7,862,323.